Method for cleaning stainless steel



3 280,038 METHOD FOR CLEANING STAINLESS STEE Earl F. Morris, Tulsa, Okla., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Mar. 20, 1964, Ser. No. 353,568 5 Claims. (Cl. 252-101) This invention concerns an improved method for the removal of mill scale and welding oxides from chromiumcontaining stainless steel articles.

This application is a continuation-in-part of my copending application, Serial Number 140,997, filed September 27, 1961, now abandoned.

Scales and oxides on articles of chromium-containing stainless steel, on subjecting them to the action of hydrofluoric acid, are initially removed, but there is the disadvantage that a protective film of black smut is formed preventing further action on the scale by the acid so that cleaning is not complete.

Nitric acid alone will not clean stair less steel. But, the addition of nitric acid to hydrofluoric acid inhibits or prevents the formation of smut." Thus, this combination This combination of acids has the marked disadvantage of being highly corrosive toward stainless steel. The corrosive action is of two types.. The first is a general overall loss of metal which will be referred to hereinafter as corrosion. The second type of corrosive reaction is peculiar to stainless steels that have been subjected to high temperature treatment. When a stainless steel has been subjected to a high temperature treatment, particularly a localized type of treatment such as welding, a temperature gradient occurs in close proximity to the heated area. In the area of high temperature, the carbon and chromium in the stainless steel react to form chromium carbide (Cr C This reaction depletes the area of chromium and the local passivity is reduced. This chromium-depleted area is subject to excessive corrosive and intergranular attack by a hydrofluoric-nitric acid mixture such as is commonly used in cleaning stainless steels. This intergranular penetration is the second type of corrosion dealt with by this invention.

It is the principal object of this invention to provide a method for effectively cleaning chromium-containing stainless steel articles without undesirable smut formation and with a minimum of corrosion and intergranular penetration of said steel articles by the cleaning composition.

These objects are attained in accordance with the invention by treatment of the chromium-containing stainless steel article which is to be cleaned with an aqueous acidic cleaning composition comprising fluoride ions from a solubl fluoride, nitric acid, and anions of an acid which is formed by the hydrolysis of a metal oxide. Hereafter such anions will be referred to as acid anions."

For compounding the aqueous acidic cleaning composition in accordance with this invention, the fluoride ion constituent may be derived from various sources as by dissolving in water to form the desired solution a soluble fluoride, e.g., hydrofluoric acid, ammonium bifluoride, sodium fluoride, sodium acid fluoride, potassium fluoride, potassium acid fluoride or mixtures thereof.

The acid anion constituent of the cleaning composition is provided by dissolving in it any hydrolyzable metal oxide or metal salt yielding in the aqueous solution an acidic anion. Examples are: manganic, permanganic, ferric, chromic, molybdic, tungstic, uranic, vanadic, niobic, tantal-ic acids; and the salts and anhydrides of these acids, which are soluble in aqueous acidic media.

The aqueous acidic cleaning composition of this invention is operable over a wide range of constituent concentrations. The concentration of fluoride ions may range H llnited States Patent 0 3,280,038 Patented Oct. 18, 1966 the other constituents of the aqueous acidic cleaning composition, and on the composition of the steel to be cleaned. Generally, acid anion" is employed in a concentration of from about 1 to about 6 percent by weight.

A simple corrosion test will determine the optimum acid anion concentration. Sample coupons of the steel to be cleaned are selected and prepared for testing by pickling for 15 minutes in a 15 percent hydrochloric acid solution, scrubbing with soap, and rinsing with water. The sample coupons are then dried with acetone, weighed, and measured to determine surface area.

A cleaning composition is then prepared for use at the desired operating temperature. The concentration of fluoride ion and nitric acid are selected within the ranges tions up to about 6 percent by weight. A portion of the bath containing no acid anion" is set aside for use as a blank in the test. Thus. a series of baths. at constant temperature, constant fluoride ion concentration, and constant nitric acid concentration, with acid anion" concentrations from zero to about 6 percent are obtained. The choice of increment size of the acid anion" concentrations will vary. If the temperature is relatively low, a smaller increment over a shorter total range is best.

The sample coupons are immersed in their respective test baths for a suitable period of time, generally about 6 hours. After the immersion period, the sample coupons are removed from their baths and rinsed. They are then dried and weighed to determine weight loss.

The corrosion rate of each bath is calculated in the following manner Corrosion rate (lbs/sq. t'tJday) Weight loss (grams) X49.l5 Surface area (sq. cm.) Xtime (hrs) The corrosion rates are then plotted against acid anion concentration. The optimum acid anion" concentration may then be read directly from the curve.

The main purpose and function of the acid anion" in the cleaning composition is to prevent "smut" and to inhibit corrosion and intergranular penetration. The cleaning power of the solution is not greatly affected by the presence of the acid anion. Thus, the optimum concentration of acid anion is determined by the corrosiveness of the solution.

In tests similar to those described above, it was found that at low concentrations of "acid anion," the corrosiveness of the solution is increased over that of the blank solution. At successively higher concentrations of "acid anion the corrosiveness increases to a peak, after which it drops off sharply and then levels off at an optimum value.

For example, tests were run as above on A151 304 stainless steel in an aqueous solution comprising 4 percent hydrofluoric acid, 25 percent nitric acid and varying concentrations of chromic acid. At F., the peak rrosion rate was found at a chromic acid conccntrati of 0.4 percent. The corrosion rate was about one and one half times that of the blank solution. At a chromic acid concentration of 1 percent, the corrosion rate of the solution dropped to about one third that of the blank solution. Raising the concentration of chromic acid to 2 percent resulted in a solution having a corrosion rate of about one fifth that of the blank solution. A solution containing 6 percent chromic acid had a corrosion rate of about one tenth that of the blank solution.

Thus, an optimum concentration range for chromic acid under these conditions would be from about 1 to about 3 percent. Less chromic acid would not give much protection from corrosion, and more would be expensive because of the low decrease in corrosiveness of the solution per unit increase in chromic acid concentration.

In substantially the same manner as described hereinbefore employing chromic acid as the source of acid anion," solutions containing fluoride ions in a concentration of from about 1 to about 50 percent by weight, anions selected from the group consisting of anions of manganic, permanganic, ferric, molybdic, tungstic, ur 'c, vanadic, niobic, and tantalic acids, in a concentrat on of from about 1 to about 6 percent by weight, and nitric acid in a concentration of from about 1 to about 70 percent by weight may be employed to clean chromium-containing stainless steels with good corrosion and intergranular penetration characteristics and with substantially no smut formation.

In carrying out the cleaning operation, the articles to be cleaned may be immersed in the cleaning solution and held therein until the desired cleaning effect is observed. Another mode of application is to flow the cleaning solution over the surface of the article to be cleaned. Agitation of the cleaning solution may be desirable. The cleaning composition of the invention exhibits a corrosion rate under dynamic conditions similar to the corrosion rate under static conditions such as would be found in an immersion treatment in an unagitated bath. The dura tion of the contact of the cleaning solution with the articles to be cleaned varies with the extent of the cleaning to be done and the temperature at which the cleaning bath is used. Immersion times of from 2 to 8 hours may be used, 6 hours being generally satisfactory with bath temperatures of 75 to 140 F. although other times and temperatures may be used. Temperatures of 80 to 120 F. are generally satisfactory.

The acidic cleaning compositions with which the present invention is concerned, generally operate by an undercutting action. The cleaning composition moderately attacks the metal beneath the scale or oxide, with a consequent sloughing off of the scale or oxide. Treatment with the acidic cleaning composition is complete when the scale or oxide is either completely removed, or loosened to a degree such that it may be easily removed by physical means. Such physical removal may be incorporated in the rinse step which follows the treatment with the cleaning composition. After rinsing, the clean article is dried and the operation is complete.

Cleaning compositions of the type disclosed by this invention are best suited to the stainless steels which have a relatively high chromium content, e.g., at least 16 percent chromium.

Permanganates bro i ichro ates used in accordance with the invention are preferred sources of the acid anion."

The aqueous acidic cleaning compositions disclosed by this invention also showed good intergranular penetration inhibition properties. Generally, the same optimum acid anion" concentrations, as determined by general corrosion tests, are operable. It is to be noted that the higher temperatures of the range disclosed will increase intergranular penetration to a much greater degree than it will increase general corrosion. Thus, if the stainless steel to be cleaned has welded areas, the lower cleaning temperatures of the range disclosed should be selected.

Various modifications may be made in the present invention without departing from the spirit or scope thereof, and it is to be understood that I limit myself only as defined in the appended claims.

I claim:

1. A method of cleaning chromium-containing stainless steel containing at least about 16 percent by weight of chromium and having a chromium-depleted area thereon comprising:

(a) contacting said stainless steel with an aqueous cleaning composition consisting essentially of an aqueous solution containing fluoride ions of a soluble fluoride in a concentration of from about 1 to about 50 percent by weight; anions selected from the group consisting of the acidic anion of one of manganic, permanganic, ferric, chromic molybdic, tungstic, uranic, vanadic, niobic, and tantalic acids, in a concentration of from about 1 to about 6 percent by weight; and nitric acid in a concentration of from about 1 to about 70 percent by weight, said aqueous cleaning composition being maintained in contact with said stainless steel until the removal of surface oxides is substantially complete and without substantial intergranular penetration of said chromium-depleted area, and

(b) rinsing the so-treated stainless steel with water.

2. A method of cleaning chromium-containing stainless steel containing at least about 16 percent by weight of chromium and having a chromium-depleted area thereon comprising:

(a) contacting said stainless steel with an aqueous cleaning composition consisting essentially of a water solution containing from about 1 to about 50 percent by weight hydrofluoric acid, from about 1 to about 6 percent by weight chromic acid and from about 1 to about 70 percent by weight nitric acid, said aqueous cleaning composition being maintained in contact with said stainless steel until the removal of surface oxides is substantially complete and without substantial intergranular penetration of said chromium-depleted area, and

(b) rinsing the so-treated stainless steel with water.

3. A method of cleaning chromium-containing stainless steel containing at least about 16 percent by weight of chromium and having a chromium-depleted area thereon comprising:

(a) contacting the stainless steel with an aqueous cleaning composition consisting essentially of an aqueous solution containing hydrofluoric acid in a concentration of from about 1 to about 50 percent by weight, permanganic acid in a concentration of from about 1 to about 6 percent by weight, and nitric acid in a concentration of from about 1 to about 70 percent by weight, said aqueous cleaning composition being maintained in contact with said stainless steel until the removal of surface oxides is substantially complete and without substantial intergranular penetration of said chromium-depleted area, and

(b) rinsing the so-treated stainless steel with water.

4. A method of cleaning chromium-containing stainless steel containing at least about 16 percent by weight of chromium and having a chromium-depleted area thereon comprising:

(a) contacting said stainless steel with an aqueous cleaning composition consisting essentially of an aqueous solution containing fluoride ions of a soluble fluoride in a concentration of from about 1 to about 50 percent by weight, anions selected from the group consisting of the acidic anion of one of manganic, permanganic, ferric, chromic, molybdic, tungstic, uranic, vanadic, niobic, and tantalic acids, in a concentration of from about 1 to about 6 percent by weight, and nitric acid in a concentration of from about 1 to about 70 percent by weight, said aqueous cleaning composition being maintained in contact with said stainless steel for from 2 to about 8 hours;

(b) physically dislodging loosened surface oxides from the stainless steel; and

(c) rinsing the stainless steel with water.

5. The method as in claim 4 in which physical dislodging of loosened oxides and rinsing are carried out concurrently as one step.

References Cited by the Examiner UNITED STATES PATENTS 3,010,854 11/ 1961 Satterfield 252-142 XR 3,082,137 3/1963 La Boda et al. 252-793 XR 3,106,499 10/ 1963 Kendall 25279.3 XR

5 LEON D. ROSDOL, Primary Examiner.

JULIUS GREENWALD, ALBERT T. MEYERS,

Urban. Examiners H l t 252-101 XR 11:15:21 252 10 XR M. WEINBLA'IT, Assistant Examiner.

McFarland 252-101 

1. A METHOD OF CLEANING CHROMIUM-CONTAINING STAINLESS STEEL CONTAINING AT LEAST ABOUT 16 PERCENT BY WEIGHT OF CHROMIUM AND HAVING A CHROMIUM-DEPLETED AREA THEREON COMPRISING: (A) CONTACTING SAID STAINLESS STEEL WITH AN AQUEOUS CLEANING COMPOSITION CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION CONTAINING FLUORIDE IONS OF A SOLUBLE FLUORIDE IN A CONCENTRATION OF FROM ABOUT 1 TO ABOUT 50 PERCENT BY WEIGHT; ANIONS SELECTED FROM THE GROUP CONSISTING OF THE ACIDIC ANION OF ONE OF MANGANIC, PERMANGANIC, FERRIC, CHROMIC, MOLYBDIC, TUNGSTIC, URANIC, VANADIC, NIOBIC, AND TANTALIC ACIDS, IN A CONCENTRATION OF FROM ABOUT 1 TO ABOUT 6 PERCENT BY WEIGHT; AND NITRIC ACID IN A CONCENTRATION OF FROM ABOUT 1 TO 70 PERCENT BY WEIGHT, SAID AQUEOUS CLEANING COMPOSITION BEING MAINTAINED IN CONTACT WITH SAID STAINLESS STEEL UNTIL THE REMOVAL OF SURFACE OXIDES IS SUBSTANTIALLY COMPLETE AND WITHOUT SUBSTANTIAL INTERGRANULAR PENETRATION OF SAID CHROMIUM-DEPLETED AREA, AND (B) RINSING THE SO-TREATED STAINLESS STEEL WITH WATER. 